Arrange oven rack 6 inches from heat source, heat broiler. Whisk eggs, water, and sale in a bowl. Melt remaining tablespoon butter in skillet, pour eggs over squash. Reduce heat to medium-low, cover, and cook until set on bottom and edges but the top is still loose, about 3 minutes. Sprinkle Gruyere evenly over the top.

Broil frittata until just set, about 1 minute. Cut into wedges to serve.